The present invention relates generally to magnetic coatings or films, and more particularly to magnetic coatings or films for use in both radar absorption and magnetic recording.
With respect to the area of magnetic recording, in order to increase the density of information stored on discs or tapes, new materials are required. Ideally the material should have a high saturation magnetization, large coercive force, and contain a large number of particles per unit volume n. The latter property is desirable since the signal-to-noise characteristics are improved if this is the case. Specifically, the signal voltage depends on the number of particles per unit volume, n, and the noise voltage depends on the square root of n. However, it is not possible to make the particles arbitrarily small, because they then act as super-paramagnets and the material has zero coercive force.
Current magnetic recording materials are primarily either alloys of Fe, Co, and Cr or their oxides. At present the oxides have been used more than metal films. In part this is because they are corrosion resistant. Metal films may be used more in the future because they have higher saturation magnetizations. The metal alloy films have saturation magnetizations of 500 to 1000 emu/cc and coercive forces of 350 to 600 Oe. Hard discs currently consist of 1,500 bits per inch along a track and 800 to 900 tracks per inch. The density of bits along the track is limited by the materials currently available.
There is considerable current interest in perpendicular recording as a means of increasing the bit density. However, the material's magnetization must be greater than 200 emu/cc and the necessary field gradients must be 1550 emu/micron. At present the bit density in perpendicular recording is limited by the grain size.
With respect to the area of radar absorption, magnetic thin film materials are required which do not reflect incident electromagnetic microwave radiation. In general, metals act as a mirror to incident radiation because they are good conductors. For a good conductor, the electric E field is zero at the surface. When an RF wave is incident on the conductor's surface, it must necessarily generate an electric field at the surface which is equal in magnitude but opposite in direction to the incident field so that the E fields cancel to maintain the E field zero at the surface. This new electric field is the electric part of the reflected wave. In order to avoid such wave reflection, it is necessary that the material have a large penetration depth so that the wave can proceed into the material where it can be attenuated or cancelled prior to exiting the material. This large penetration depth is accomplished by designing the material so that a large majority of the magnetic particles making up the material do not touch one another to form conducting paths through the material. When the magnetic particles comprising the material do not touch on average, the resistance of the material is increased.
Materials with large penetration depths can absorb radiation if they also have a large magnetic permeability, .mu.", for a desired bandwidth, where .mu." is the imaginary part of the magnetic permeability which accomplishes the absorption of the microwave radiation. In this regard, the magnetic field generated by the material in response to microwave radiation which is in-phase with this radiation is proportional to the real part, .mu.' of the magnetic permeability. Usually the reflected wave is minimized if .mu.'=.epsilon., where .epsilon. is the dielectric constant for the material. The imaginary part of the magnetic field is proportional to .mu." and is 90.degree. out of phase from the real part.
There is a need for high-temperature radar-absorbing materials because of the elevated temperatures of certain plane surfaces such as the leading edges of air foils and jet engine inlets and exhausts. To be useful as an engine exhaust RAM, for example, the material must function at 400.degree.-600.degree. C. and be able to survive the higher temperatures, 1100.degree.-1200.degree. C., that occur when the afterburner is on.
General Electric, in Contract Report No. AFAL-TR-82-1040, generated under contract F33615-80-C-1094 discloses a process for preparing Fe particles coated with Al.sub.2 O.sub.3 for use as a radar absorbing material. In this process FeAl or FeCoAl alloys are prepared by means of inert gas atomization. These particles are then graded by size and particles of several microns in diameter are chosen for use to obtain best results.
The particles are heated so that the Al diffuses to the surface and then this surface coating of Al is oxidized. These particles are then put in a binder and the material is coated on a surface. Approximately 50 volume % of the composite is Fe. On the basis of the information available, this process has the following disadvantages:
1. The Fe particles are too large to be single domain. Thus, there is a broad multidomain resonance with much of the intensity above the "conventional" 2-18 GHz threat band. PA1 2. The Al which does not diffuse out of the Fe lowers the Curie point approximately 50.degree. C. This lowers the upper limit of usefulness of the material. PA1 3. It is difficult to find a suitable binder which can withstand high temperatures, does not attack the aluminia coating the Fe, and also is capable of bonding to a given surface. PA1 4. The process is complicated and expensive.